Group-III nitride devices and systems on IBAD-textured substrates

ABSTRACT

A multilayer structure including a hexagonal epitaxial layer, such as GaN or other group III-nitride (III-N) semiconductors, a &lt;111&gt; oriented textured layer, and a non-single crystal substrate, and methods for making the same. The textured layer has a crystalline alignment preferably formed by the ion-beam assisted deposition (IBAD) texturing process and can be biaxially aligned. The in-plane crystalline texture of the textured layer is sufficiently low to allow growth of high quality hexagonal material, but can still be significantly greater than the required in-plane crystalline texture of the hexagonal material. The IBAD process enables low-cost, large-area, flexible metal foil substrates to be used as potential alternatives to single-crystal sapphire and silicon for manufacture of electronic devices, enabling scaled-up roll-to-roll, sheet-to-sheet, or similar fabrication processes to be used. The user is able to choose a substrate for its mechanical and thermal properties, such as how well its coefficient of thermal expansion matches that of the hexagonal epitaxial layer, while choosing a textured layer that more closely lattice matches that layer. Electronic devices such as LEDs can be manufactured from such structures. Because the substrate can act as both a reflector and a heat sink, transfer to other substrates, and use of external reflectors and heat sinks, is not required, greatly reducing costs. Large area devices such as light emitting strips or sheets may be fabricated using this technology.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 15/041,017, entitled “Epitaxial Hexagonal Materialson IBAD-Textured Substrates”, filed on Feb. 10, 2016 and issuing on Aug.15, 2017 as U.S. Pat. No. 9,735,318, which application claims priorityto and the benefit of the filing of U.S. Provisional Patent ApplicationSer. No. 62/114,504, entitled “IBAD-Textured Substrates for Growth ofEpitaxial Group-III Nitride Materials and Method of Making the Same”,filed on Feb. 10, 2015, and U.S. Provisional Patent Application Ser. No.62/262,815, entitled “IBAD-Textured Substrates for Growth of EpitaxialGroup-III Nitride Materials and Method of Making the Same”, filed onDec. 3, 2015. The specification and claims thereof are incorporatedherein by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of AssistanceAgreement No. AR0000447 awarded by the U.S. Department of Energy'sAdvanced Research Projects Agency-Energy.

BACKGROUND OF THE INVENTION Field of the Invention (Technical Field)

The present invention relates epitaxial growth of a layer of a hexagonalmaterial, such as gallium nitride (GaN) or other group III-nitride(III-N) semiconductors, on a substrate whose crystalline alignment isformed by the ion-beam assisted deposition (IBAD) texturing process. Inone embodiment, IBAD textured layers are used to prepare biaxiallyaligned thin films or substrates which are single-crystal-like innature. These IBAD thin films or templates support subsequent depositionof optional epitaxial buffer layers followed by GaN or III-N epitaxialgrowth. An electronic component that includes III-N epitaxy on anion-beam textured layer with intermediate epitaxial buffer layers on topand a method of forming the same are disclosed.

An embodiment of the present invention is an ion beam assisteddeposition (IBAD) texturing process for biaxially aligned films astemplates for GaN epitaxy. The IBAD process enables low-cost,large-area, flexible metal foil substrates to be used as potentialalternatives to single-crystal sapphire and silicon for electronicdevices. Epitaxial GaN films are grown by the MOCVD process on theseengineered flexible substrates, which enables scaled-up roll-to-roll,sheet-to-sheet, or similar fabrication processes to be used. GaN filmshaving a thickness of several microns on polycrystalline metal foilsthat have in-plane and out-of-plane alignment of less than 1° have beenmanufactured. The epitaxial GaN films on polycrystalline metal foil areused as a template layer to make multi-quantum well light emitting diode(LED) structures and have successfully demonstratedelectro-luminescence. These are the first LED devices fabricateddirectly on metal foil, and can be scaled up using a roll-to-rollprocess. Thus epitaxial-GaN layers, i.e. single-crystal-like qualitymaterial, can be deposited directly (with no transfer) on top ofpolycrystalline flexible metal foils by use of intermediate ion-beamassist deposited (IBAD) textured layers. Such epi-GaN layers can then beused as buffer layers for GaN-based device structures.

Background Art

Note that the following discussion may refer to a number of publicationsand references. Discussion of such publications herein is given for morecomplete background of the scientific principles and is not to beconstrued as an admission that such publications are prior art forpatentability determination purposes.

Light-emitting diodes (LEDs) are revolutionizing the way the world isimplementing lighting at the start of the 21st century. Not only areLEDs more efficient light sources, but they have the ability to beimplemented in many different forms compared to other light sources andto have their spectrum adjusted for application as well as modified intime. However, the greatest barrier still holding back LEDs fromcompletely replacing incandescent and fluorescent lighting is cost ofthe LED luminaire systems. Although LED lighting has made great stridesin penetrating the lighting market, it is currently mostly focused inthe niche high-end lighting space and still far from a mainstreamapplication in commercial lighting where it is hard to compete in costwith simple fluorescent tubes. For LEDs to dominate the whole lightingmarket the cost of LED lighting will still have to come down by severalorders of magnitude. This is in spite of the fact that the LED chip andpackage costs have already come down incredibly by several orders ofmagnitude in the past decade. The cost of packaged LEDs today can evenbe less than $0.50/klm, compared to an average LED package price of$50/klm a decade ago. There is still room to go in reducing cost byanother factor of 2 or 3 using current fabrication techniques. To gosignificantly further in cost reduction one has to tackle the criticalissue of scale in manufacturing of the LED chips and packages as well asreduction in the cost of other parts of the luminaire system. Thepackaged LEDs are used as surface mounted devices (SMD) and typicallyimplemented with a pick-and-place (P&P) technology in lighting devices.P&P machines are automated ways of mounting SMDs mechanically.Eliminating SMD's and P&P would simplify the LED luminaire considerablyand reduce costs. The way semiconductor industry does scale up ofsemiconductor chips is to increase the substrate size incrementally from2-inch to 4-inch and now going to 6-inch single-crystal wafers. Most ofblue LED production today is done using a GaN platform on sapphire.High-quality epitaxial GaN is deposited usually by metal-organicchemical vapor deposition on sapphire (MOCVD, sometimes called OMVPE ororgano-metallic vapor phase epitaxy) and then used as a platform forsubsequent deposition of epitaxial device structures.

GaN and related group III-N materials are used for numerousapplications, including light emitting diodes (LEDs), laser diodes(LDs), and transistor devices such as high-electron mobility transistors(HEMTs). The vast majority of today's GaN layers are depositedepitaxially on single-crystal substrates such as sapphire, silicon,silicon carbide, or gallium nitride. However, single-crystal substratesare typically rigid, expensive, and readily available in diameters ofonly less than 100 mm, except for silicon wafers. An exception tosingle-crystal substrates is the development of ion-beam assisteddeposition (IBAD) of single-crystal like thin films on flexiblesubstrates. In the last decade, IBAD texturing of thin films on flexiblemetal has been notably been developed for long lengths ofsuperconducting crystals for electrical wire applications.

When typically growing GaN using MOCVD (metal-organic chemical vapordeposition) on single-crystal substrates that are not native (i.e.heteroepitaxially), a two-step deposition process is used whereby aninitial GaN nucleation layer (NL) is deposited at a relatively lowtemperature (500-600° C.) to facilitate GaN nucleation and evolution. Inthe second step, the fully coalesced epitaxial GaN layer is then grownat a higher temperature (>1000° C.) to obtain device quality GaNmaterial on top of the NL. Limiting the growth of GaN to lattice-matchedsingle-crystal substrates reduces the number of practical substrates tosapphire (Al₂O₃), SiC, and bulk-GaN, which can be expensive andunavailable in large sizes. More recently Si has been developed as asingle crystal substrate for GaN epitaxy and is becoming morewidespread. Despite the adoption of Si as a potential alternativesubstrate to sapphire, direct growth of GaN on metal and othersubstrates is desired for practical applications that need large area orflexible substrates. Thus far it has not been possible to growsingle-crystal GaN directly on metal or other non-oriented substratesdue to the lack of epitaxial registry. GaN on metal or othernon-oriented substrates has been achieved by transfer of the grownepitaxial GaN layer onto the foreign substrate, or by transferring anoriented film such as graphene and growing GaN on top of the graphene.

Previous IBAD texturing in fluorites was not developed, not easy to workwith, and the IBAD texture widths were more than 15° in-plane FWHM. ThusIBAD (111) was not thought to be of sufficient quality to producehigh-quality semiconductor materials with in-plane alignment of <1°. Thebest semiconductor Si results on (111) IBAD had in-plane texture FWHMof >10° and out-of-plane texture 1.5°. Thus typical semiconductormaterials on IBAD are of inferior quality for devices and cannot competewith semiconductors on single crystal substrates. Good quality LED andother devices have not been produced. Several previous attempts to makeGaN devices have been unsuccessful, since the materials have not been ofhigh enough quality in terms of crystalline perfection and carriermobility.

LEDs are revolutionizing lighting in the world due to their increasedefficiency compared to other light sources and their spectrumtunability. However, LEDs are difficult to scale up further inmanufacturing. Currently InGaN LEDs are produced primarily on 4-inch and6-inch sapphire wafers and then diced into small dies. Larger sapphirewafers have significant issues in manufacturing due to a CTE mismatchand consequent substrate bowing. LED manufacturing cost is thereforelikely not to decrease much further in the current paradigm ofGaN-on-sapphire due to a lack of suitable scale up. In addition, LEDcosts are dominated by relatively expensive epi layers. The epi cost isstill dominated by substrate cost and lack of suitable scale up.Therefore devices are limited to small areas and LEDs are packaged asindividual devices and used as surface mounted devices (SMD). Thisrequires complicated pick-and-place (P&P) mechanical machinery and lackscompact integration. Furthermore, backend wafer processing and packagingof LEDs is rather complex involving dozens of steps, contributingsignificantly to LED cost. About ¼ of an LED package cost is in backendprocessing and ½ is in packaging. Finally, LED performance is hinderedby the existence of “droop,” loss of efficiency at high operatingcurrent density. In order to maximize light output per unit areamanufacturers of devices are required to inject high current densitiesin LEDs which limits their performance due to efficiency droop. Typicaloperating points for high brightness LEDs are around 30 A/cm² wherethere is a 10-20% reduction in efficiency. Furthermore, LEDs exhibit athermal droop, i.e. a reduction in optical power when temperature isincreased. At a 100° C. this is typically a 10-20% reduction.

SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION)

The present invention is a multilayer structure comprising an epitaxialhexagonal crystal layer, a layer of a cubic material having a <111> outof plane orientation and having an in-plane crystalline texture with afull width half maximum (FWHM) less than or equal to approximately 15°,and a non-single crystal substrate. The epitaxial hexagonal crystallayer preferably comprises a group III-nitride semiconductor such asGaN. The epitaxial hexagonal crystal layer preferably serves a templatelayer for a light-emitting diode (LED). The layer of cubic materialpreferably has been textured by ion beam-assisted deposition (IBAD). Thesubstrate can be amorphous, polycrystalline, flexible, ductile,metallic, ceramic, glass, plastic, or polymer. The epitaxial hexagonalcrystal layer was preferably grown using metal-organic chemical vapordeposition (MOCVD), reactive sputtering, reactive evaporation, ormolecular beam epitaxy (MBE). The coefficients of thermal expansion ofthe substrate and the epitaxial hexagonal crystal layer are preferablywithin approximately 12%, and more preferably within approximately 5%.If the epitaxial hexagonal crystal layer comprises GaN the substratepreferably comprises molybdenum, tungsten, tantalum, alloys thereof,Mo—Cu, or TZM. The layer of cubic material preferably has an in-planecrystalline texture having a FWHM of less than or equal to approximately12°, or more preferably less than or equal to approximately 8°, or evenmore preferably less than or equal to approximately 5°. The layer ofcubic material preferably comprises MgO, CeO₂, a bixbyite structure,Sc₂O₃, Y₂O₃, Al₂O₃, a fluorite structure, TiN, a rock salt structure,CaF₂, cubic ZrO₂, HfO₂, ScO_(x), or Mn₂O₃. The structure preferablycomprises a base layer disposed between the substrate and the layer ofcubic material. The base layer preferably comprises amorphous Al₂O₃,Y₂O₃, or SiO₂. The structure preferably comprises one or more epitaxialbuffer layers disposed between the layer of cubic material and theepitaxial hexagonal crystal layer. The epitaxial buffer layerspreferably each have a lattice parameter that successively provides atransition from the lattice parameter of the cubic material to thelattice parameter of the epitaxial hexagonal crystal. If the epitaxialhexagonal crystal layer comprises GaN the epitaxial buffer layerspreferably comprise a layer of Sc₂O₃ and a layer of Zr, and a layer ofAlN. The FWHM of the in-plane texture of the layer of cubic material isoptionally greater than an FWHM of an in-plane texture of the epitaxialhexagonal crystal layer. The invention is also an electronic oroptoelectronic device comprising the multilayer structure of claim 1,such as an LED, MOSFET, MESFET, HEMT, Heterojunction FET, heterojunctionbipolar transistor (HBT), thin-film transistor, sensor, memristor, laserdiode (LD), SAW device, spintronic device, photodetector, orphotovoltaic (PV) diode.

The present invention is also an electronic or optoelectronic devicecomprising a substrate on which an active region of the device wasgrown, said substrate acting as a reflector. The device can be an LED,MOSFET, MESFET, HEMT, Heterojunction FET, heterojunction bipolartransistor (HBT), thin-film transistor, sensor, memristor, laser diode(LD), SAW device, spintronic device, photodetector, or photovoltaic (PV)diode. The substrate preferably is a heat sink and has a thermalconductivity greater than approximately 25 W/m·K, more preferablygreater than approximately 50 W/m·K. The substrate preferably comprisesa metal or alloy, is preferably flexible, and is preferably not a singlecrystal. The device preferably comprises a light emitting region whichis two-dimensional and not a point source, such as a sheet or strip. Thedevice preferably comprises an LED on a metal substrate, wherein anoperating temperature of the LED is less than approximately two thirds,and more preferably less than approximately one half, that of an LED onsapphire so that it is cool to touch during operation. The device ispreferably integrated into an electronic system such as an LED-basedluminaire, a light emitting strip, a light emitting sheet, an opticaldisplay, or a MicroLED display.

The device preferably comprises an epitaxial hexagonal crystal layer anda layer of a cubic material having a <111> out of plane orientation andhaving an in-plane crystalline texture with a full width half maximum(FWHM) less than or equal to approximately 15°. The epitaxial hexagonalcrystal layer preferably comprises a group III-nitride semiconductor,preferably GaN. The layer of cubic material has preferably been texturedby ion beam-assisted deposition (IBAD) and is preferably selected fromthe group consisting of MgO, CeO₂, a bixbyite structure, Sc₂O₃, Y₂O₃,Al₂O₃, a fluorite structure, TiN, a rock salt structure, CaF₂, cubicZrO₂, HfO₂, ScO_(x), and Mn₂O₃. The epitaxial hexagonal crystal layerpreferably comprises GaN and the substrate comprises molybdenum,tungsten, tantalum, alloys thereof, Mo—Cu, or TZM. The layer of cubicmaterial preferably has an in-plane crystalline texture having a FWHM ofless than or equal to approximately 12°, more preferably 5°. The devicepreferably comprises a base layer disposed between the substrate and thelayer of cubic material, the base layer optionally comprising amorphousAl₂O₃, Y₂O₃, or SiO₂. The device also preferably comprises one or moreepitaxial buffer layers disposed between the layer of cubic material andthe epitaxial hexagonal crystal layer. The epitaxial buffer layerspreferably each have a lattice parameter that successively provides atransition from the lattice parameter of the cubic material to thelattice parameter of the epitaxial hexagonal crystal layer. When theepitaxial hexagonal crystal layer comprises GaN the epitaxial bufferlayers preferably comprise a layer of Sc₂O₃, a layer of Zr, and a layerof AlN.

Objects, advantages and novel features, and further scope ofapplicability of the present invention will be set forth in part in thedetailed description to follow, taken in conjunction with theaccompanying drawings, and in part will become apparent to those skilledin the art upon examination of the following, or may be learned bypractice of the invention. The objects and advantages of the inventionmay be realized and attained by means of the instrumentalities andcombinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the specification, illustrate the practice of embodiments of thepresent invention and, together with the description, serve to explainthe principles of the invention. The drawings are only for the purposeof illustrating certain embodiments of the invention and are not to beconstrued as limiting the invention. In the drawings:

FIG. 1 is a schematic diagram showing an epitaxial GaN layer on top ofan IBAD-textured layer and intermediate epitaxial buffer layers. TheIBAD layer is prepared on a smooth surface that is obtained byplanarizing a metal substrate, for example a metal foil.

FIGS. 2A and 2B are Reflection High-Energy Electron Diffraction (RHEED)images of IBAD-MgO and homo-epi MgO layers, respectively.

FIG. 3 is an x-ray diffraction (XRD) (202) pole figure of an MgO filmcreated on an IBAD-MgO film on metal tape.

FIGS. 4A and 4B are RHEED images of epi-CeO on IBAD-CeO, and epi-GaN ontop of buffered IBAD-CeO, respectively.

FIG. 5 is an XRD theta-2theta scan in GADDS area detector, showing thebright spot for GaN (002) reflection. The rings are from the underlyingmetal substrate.

FIG. 6 is an XRD (101) pole figure for the GaN film on γ-Al₂O₃ bufferedmetal tape.

FIG. 7 is an x-ray diffraction pole figure of a CeO₂ film, (220) pole,created on an IBAD-CeO film on metal tape.

FIG. 8 is an x-ray diffraction (101) pole figure of a GaN film createdon an IBAD-CeO film on metal tape.

FIGS. 9A-9C are SEM images of epi-GaN films on IBAD/metal tape, showinga progression of island coalescence from FIG. 9A to FIG. 9C.

FIGS. 10A and 10B are optical images of GaN layers on the IBAD templateon metal foil. Thin (approximately 100 nm) AlN (FIG. 10B) or AlN/GaN(FIG. 10A) were deposited by physical vapor deposition (PVD) and used asseed layers for MOCVD GaN growth.

FIG. 11 is a TEM cross-section of an epi-GaN film on top of metal tapehaving a planarizing layer (SDP) and IBAD textured layer on top.

FIG. 12 is a RHEED image during IBAD deposition of cerium oxide (111).

FIG. 13 is a RHEED image after deposition of homoepitaxial cerium oxideon IBAD-cerium oxide (111).

FIG. 14 is a RHEED image during IBAD deposition of scandium oxide (111).

FIG. 15 is a cross-sectional TEM image of epi-GaN layer deposited on theIBAD template on a metal substrate.

FIG. 16 shows LED luminescence from an LED structure deposited on anIBAD template on a flexible metal substrate.

FIG. 17 shows an X-ray diffraction (XRD) (220) pole figure of abiaxially aligned CeO₂ film created on an IBAD-CeO film on metal tape.The ion beam is directed from the left.

FIGS. 18A-18B are SEM images of epi-GaN films on IBAD/metal tape atdifferent magnification.

FIG. 19A is a RHEED image of an IBAD-Sc₂O₃ film after deposition with(111) out of plane orientation (replaces FIG. 14).

FIG. 19B is a RHEED image after deposition of a homoepitaxial Sc₂O₃layer on top of the IBAD-Sc₂O₃.

FIG. 20 is an X-ray theta-2theta scan of the Sc₂O₃ film oriented by IBADtexturing on a polycrystalline metal foil.

FIG. 21 is an X-ray pole figure for (440) peak in the Sc₂O₃ biaxiallyoriented film.

FIG. 22 is a schematic of the SDP process for planarizing a substrate.

FIG. 23 shows experimental results of planarization.

FIG. 24 is a comparison of epitaxial structures for GaN on sapphire, Si,and IBAD <111>.

FIG. 25A is a transmission electron micrograph of an LED structure crosssection. The LED structure is shown at the top of the GaN layer.

FIG. 25B is a detail of FIG. 25A.

FIG. 26 is a micrograph having a higher magnification than that of FIG.25, showing the LED multi-quantum well (MQW) structure InGaN/GaN on topof the GaN layer. The lighter regions indicate InGaN.

FIG. 27 is a graph showing photoluminescence (PL) data of LEDsfabricated on sapphire (in blue) and on an IBAD template on metal foil(in red). The PL peak for the IBAD LED is broader than the sapphire LEDand lower intensity. In this case the IBAD LED is about 15% of theintegrated PL intensity of the sapphire LED.

FIG. 28 is a graph showing the light-current (LI) curve for LED devicesfabricated on single-crystal sapphire and on an IBAD template preparedon metal foil.

FIG. 29 is a graph showing the intensity ratio for IBAD LED vs sapphireLED from the LI curves of FIG. 28.

FIG. 30 is a plan-view transmission electron micrograph of GaN-on-metalshowing threading dislocations with a density (TDD) of ˜6×10⁸/cm².

FIG. 31 is an XRD (101) pole figure of a 5 micrometer thick epitaxialGaN layer grown by the MOCVD process.

FIGS. 32A and 32B show an LED structure patterned on a LED GaN-on-metaldevice such as that shown in FIGS. 25-26. FIG. 32A shows the outline ofthe patterned 300 μm square mesa structure. FIG. 32B shows the samedevice with light emitted upon application of 5 mA. FIG. 32C shows aside-view schematic of the patterned structure shown in FIGS. 32A and32B.

FIGS. 33A and 33B are graphs of voltage-current (VI) and light-current(LI) for LED devices fabricated on single-crystal sapphire substrate andon a GaN-on-metal template, respectively.

FIG. 34 is an optical photograph of a 30 micron diameter patterned LEDstructure on a metal foil.

FIG. 35 is a schematic of an LED on a metal substrate.

FIG. 36 is an artist's rendition of a Light Emitting Strip manufacturedon a strip of metal

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention is related to an approach to scale up GaNproduction using ion-beam crystal alignment for epitaxial films. Theprocess is sometimes called ion-beam assisted deposition (IBAD), but itreally refers to texturing of films with IBAD. IBAD texturing can occurat different time scales or thicknesses of films, but preferably occursright at initial film nucleation and coalescence, which is known asion-texturing at nucleation (ITaN). It typically occurs in the first5-10 nm of deposit and has been demonstrated to be extremely fast, canoccur in less than 1 second of deposition time. IBAD texturing can beperformed on a large-area substrate that is typically a flexible metalfoil. IBAD texture formation can alternatively be performed on flexibleglass, ceramic, plastic or polymer. The process is easily scalable tolong lengths with flexible substrates that can be put onto a spoolsuitable for roll-to-roll processing. The substrate itself can bepolycrystalline but is preferably chosen for its mechanical as wellthermal properties. Thus the material and the thickness of the foil arepreferably optimized for the final application. Thin foils arepreferably flexible and ductile. Additionally, since MOCVD growth (whenutilized) is performed at high temperature, the material preferably hasa good match to the coefficient of thermal expansion (CTE) of the thickGaN layers that will subsequently be deposited. For GaN molybdenum ortungsten or alloys thereof are preferably used. By eliminating the needfor lattice matching in the substrate a greater variety of substratesmay be chosen.

Embodiments of the present invention are based on a novel GaN growthprocess which, compared to existing methods reported for GaN growth,comprises the use of non-single crystal substrates such aspolycrystalline commercial metal foils or amorphous glass, upon whichepitaxial GaN films are deposited directly and can be used for variousapplications including electronics, optics and optoelectronics, etc. Insome embodiments, several different uniform films of thicknesses in therange of tens of nm are preferably evaporated or sputtered ontobiaxially textured layers that are generated by an IBAD process.Amorphous wafers (amorphous materials such as glass, or a single-crystalwafer coated by a vacuum deposited amorphous thin film) orpolycrystalline metal substrates, for example, can be used as flatsubstrates. For metal substrates it is preferable to utilize materialshaving a coefficient of thermal expansion closely matching that of GaN,for example elements such as molybdenum, tungsten, tantalum and alloysthereof. FIG. 1 is a schematic of an embodiment of the presentinvention.

The process of putting GaN on metal typically comprises three steps: 1)SDP, 2) IBAD+buffer layers, and 3) MOCVD GaN. The first step is solutiondeposition planarization (SDP). It is essentially chemical solutiondeposition (CSD) that has been optimized to smooth, or level, thesurface by multiple coatings. In practice an initial root mean square(RMS) roughness of 20-100 nm on 5×5 μm scale can be reduced to about 0.5nm after 10-30 coatings. A schematic and results of the SDP process areshown in FIGS. 22 and 23. SDP prepares the substrate for the IBADtexturing process, especially for ITaN, for which only a thin deposit isneeded. ITaN typically requires a very smooth surface, i.e. less than 2nm RMS roughness, and the smoother texture the better. In addition tosmoothing the substrate, SDP can be adjusted to additionally provide anappropriate base layer material on which the IBAD layer is deposited.ITaN, in particular, requires appropriate chemistry in the base layerfor the ion texturing to work. SDP is typically not required if thesubstrate is sufficiently smooth.

In past decades a number of ITaN materials have been explored, but mostwork was focused on MgO. IBAD-MgO produces <100> out of planeorientation and that is suitable for depositing cubic materials on topwith the same orientation. However, for deposition of hexagonallysymmetric materials, such as wurtzite GaN, it can produce two epitaxialdomains that are 30° rotated with respect to one another. In order to gofrom a cubic material to a hexagonal structure one would need a <111>orientation, as shown in FIG. 24. This is commonly done today in growingGaN on Si, where one uses a <111> oriented Si substrate. The quality of<111> IBAD in terms of the full width half maximum (FWHM) of thein-plane orientation has not been good enough to grow GaN, and notnearly as good as IBAD-MgO. IBAD-MgO has been reported down to 1.5°FWHM, with homoepi layers. The best <111> with CaF₂ obtained has beenabout 15°. One embodiment of the present invention uses CeO₂ with a FWHMin-plane orientation of about 8°. However, by growing GaN using MOCVD,that improved to less than 1° in the GaN layer. The way that GaN growsby MOCVD, starting with small seeds that coalesce and grow into largegrains, accommodates an IBAD structure with poorer texture and stillachieves high quality materials. Thus other IBAD materials, such asbixbyite structures, may be used.

The IBAD layer can in principle be chosen to lattice match thefunctional epitaxial layer, e.g. GaN. In order to come closer to the GaNlattice an epitaxial buffer layer can be deposited that transitions fromthe 3.826 Å lattice parameter of CeO₂ to 3.189 Å of GaN. Many materialswith intermediate lattice parameters can be used. In one embodiment,Sc₂O₃ and Zr are used as two intermediate layers that will transition toAlN. A sputtered AlN surface is then used as a seed layer for GaNgrowth. That complete structure, called the IBAD template, can then beused as a template for epitaxial GaN growth. MOCVD GaN is grown directlyon AlN or on GaN deposited by physical vapor deposition (PVD) on theIBAD template.

In one embodiment a substrate comprises a metal foil, preferably havinga CTE matched to the desired functional semiconductor material.Deposited on the substrate is a base layer to enable IBAD. This layercan also be used to planarize the substrate and act as a diffusionbarrier. Next is deposited an IBAD textured layer, then one or moreintermediate buffer layer(s). The final layer is a hexagonal material,preferably a semiconductor material such as III-N or ZnO. The substrateis preferably chosen to match the functional layer or semiconductorlayer coefficient of thermal expansion as closely as possible. The IBADlayer is preferably chosen to match the lattice constant of thesemiconductor layer as closely as possible. This enables the decouplingof matching the two properties in selecting the substrate. In aconventional single-crystal substrate one has to use the materials thathave a good enough lattice match and cannot independently adjust otherproperties.

For substrates used for IBAD texturing, a substrate with desiredmechanical properties, such as flexibility and thermal properties suchas CTE (coefficient of thermal expansion) can be selected, and then theIBAD layer with the desired lattice constant can be chosenindependently. Furthermore, a large area substrate, not one that islimited to single-crystal boule sizes, can be used, enabling productionto be scaled to extremely large areas, and enabling production ofintegrated devices over those large areas via printing, for example.Roll-to-roll (R2R) is a method to scale production to very large areas(in small volumes), but it can also be scaled up sheet-to-sheet (S2S).Also, the substrate and IBAD layer can have different orientations,lattice mismatches, etc., greatly increasing the versatility of thepresent invention.

One embodiment of the present invention comprises a single-crystal-likehexagonal-structure material, epitaxially deposited on anion-beam-assist deposit (IBAD) textured layer. Epitaxial intermediatebuffer layers between the IBAD layer and the hexagonal-structurematerial may optionally be deposited. The hexagonal-structure materialoptionally comprises graphene, MoS₂, WS₂, or another two dimensionalmaterial, or GaN, AlN, InGaN, or another III-N material. The IBAD layermaterial preferably comprises a cubic structure in the (111) orientationgiving a 3-fold symmetry for alignment of the hexagonal materials ontop, such as fluorite or bixbyite structure materials.

Another embodiment of the present invention is ion-beam alignment of afilm of a bixbyite-structure material, i.e. structure of (Mn—Fe)₂O₃,such as Sc₂O₃ or Y₂O₃, on top with a (111) orientation out of plane andin-plane orientation, preferably with in-plane orientation better than15°, and more preferably with in-plane orientation better than 10°.

Embodiments of the present invention comprise a CTE-matched metalsubstrate for IBAD obtained by alloying the metal to get perfectmatching to the semiconductor, such as in the Mo—Cu alloy system.

Embodiments of the present invention comprise base (or planarization)layers for IBAD flourites or bixbyites that include amorphous-Al₂O₃,Y₂O₃, SiO_(x); these layers can be deposited by chemical solutiondeposition in manner that produces planarization (e.g. SDP).

Embodiments of the present invention comprise integration of activedevices fabricated using epitaxial III-N materials on IBAD substrates,such as metal foils with a textured layer. Devices are printed usingseveral different possible printing technologies, e.g. screen printingor inkjet printing, patterned and contacts and passive devices arepreferably printed on top afterwards. A display can be manufacturedusing this printing of LED devices on IBAD substrates. Power devicesintegrated with LEDs can provide constant power, switching, or dimmingcontrol of LED devices, or different colors and different colortemperatures.

Embodiments of the present invention are novel methods for GaN growth onnon-single-crystal substrates with the use of an IBAD template (4-foldsymmetric and 3-fold symmetric IBAD) that can be applied to almost anysubstrate that can sustain the GaN growth temperature, in the case ofMOCVD above 1000° C., such as metals, ceramics or glass (quartz). Othermethods for deposition of GaN-based devices are reactive evaporation(esp. MBE) and reactive sputtering. The latter methods utilize a lowertemperature during deposition and growth and hence are more amenable tonon-standard substrates that are not single-crystal wafers, such asplastics and glass. Matching the coefficient of thermal expansion (CTE)enables metals to be used ideally as substrates for GaN growth. Thesemetals include, for example, molybdenum, tantalum, tungsten and alloysof these elements with other elements, such as TZM, an alloy ofmolybdenum and small amounts of titanium and zirconium, ormolybdenum-copper alloys. These alloys exhibit a high thermalconductivity which is useful for devices requiring conductive coolingsuch as GaN power electronics.

Embodiment substrates for growth of epitaxial films of group-Ill nitrideformed by ion-beam textured layers with epitaxial overlayers compriseIBAD biaxially textured layers comprising IBAD-MgO, TiN, or otherrock-salt structured materials, previously known to be amenable toion-beam biaxial texturing, but also IBAD-CeO₂ (cerium dioxide) or otherfluorite structure materials such as CaF₂, cubic ZrO₂, or HfO₂, whichform a (111) orientation during IBAD. Other materials includeIBAD-ScO_(x) (with the Sc₂O₃ structure) and other oxides or nitrides inthe bixbyite structure, such as Y₂O₃ or Mn₂O₃, bixbyite being a vacancyordered derivative of the fluorite structure; epitaxial overlayers(buffer layers) lattice matched for growth of GaN or other group IIInitride compounds, such as AlN or other nitrides, or elemental metallayers such as Zr or Hf, or oxides such as cubic Al₂O₃. Asingle-crystal-like cubic film with a (111) orientation on an arbitrarysurface can be manufactured for growth of epitaxial III-N, or otherhexagonal structure semiconductor or semi-metal such as InP (111),transition metal dichalogenides, and Indium Gallium Zinc Oxide (IGZO),layers using ion beam textured layers. These same textured layers can beobtained by other means besides IBAD such as inclined substrateevaporation or inclined sputtering. III-N layers can be grown by MOCVD,MBE, reactive evaporation, reactive sputtering, or other methods. Ametal substrate may be used to produce III-N layers on a flexible metalfoil or other metal substrate by use of an ion beam textured layer;metal substrates include materials such as molybdenum, tantalum,tungsten and alloy of these elements with other elements. III-N layersmay be grown on a glass substrate with an intermediate ion beam assisteddeposition (IBAD) textured layer. An electronic or optoelectronic devicecan be manufactured that comprises the epitaxial III-N material on anIBAD template substrate; such a device includes MOSFET's, MESFETs,HEMTs, Heterojunction FETs, heterojunction bipolar transistors (HBTs),thin-film transistors, sensors, memristors, light emitting diodes(LEDs), laser diodes (LD), SAW devices, spintronic devices,photodetectors, photovoltaic (PV) diodes. Furthermore these devices canbe used in products such as LED-based displays, LED-based lightingproducts, PV cells and modules.

Some embodiments of the present invention are a process for making analigned layer on top of a metal to manufacture an LED; an LED structuremade on top of an ion-aligned layer; a metal/amorphous planarizinglayer/(111) textured layer/hexagonal semiconductor layer structure; IBADtexturing of Bixbyite materials; base layers deposited on a flexiblesubstrate for IBAD layers, such as amorphous Al₂O₃, Y₂O₃, or SiO₂; a GaNPVD layer used as a nucleation layer for MOCVD GaN; and a metal alloysubstrate to match the CTE of GaN very closely, such as Mo—Cu alloy.

The IBAD texture has been improved in embodiments of the presentinvention to below 10°, and the use of additional buffer layers and ahigh temperature MOCVD process for GaN growth produces GaN of muchhigher quality, <1° in-plane FWHM (as opposed to >10°) and less than0.5° out-of-plane (as opposed to <1.5°) that enables the manufacture ofhigh quality devices. The way that epitaxial GaN grows in embodiments ofthe present process is fundamentally different from epitaxial growth ofSi and other semiconductors. This means that active devices such as alight-emitting diode (LED) consisting of III-N materials (such as InGaN)fabricated on a (111) ion-beam-assist deposit (IBAD) textured layer canbe fabricated.

Example 1

To demonstrate the applicability of IBAD templates for epitaxial growthof GaN and related group III nitride materials, MOCVD growth of GaN,also known as MOVPE (metal-organic vapor phase epitaxy), was performed.Samples were briefly heated to 800-1000° C. in flowing H₂ prior togrowth. The GaN nucleation layer was grown using trimethylgallium (TMGa)and ammonia (NH₃) using H₂ and N₂ push flows at a substrate temperatureof 530° C. After the growth of the GaN NL, the TMGa was turned off andthe wafers were ramped in temperature to 1050° C. over 8 minutes. At1050° C. the TMGa was turned on for 1 hour resulting in ≤2 μm of GaNfilm. After growth the wafers were cooled in the NH₃, H₂, and N₂ flowsand removed from the growth reactor.

For the templates for GaN growth, substrates were prepared separatelyprior to GaN growth. In one embodiment we utilized IBAD-textured MgOfilms. These films are oriented with the (100) axis out of the plane andform a square, 4-fold symmetric lattice on the surface. IBAD-MgO filmswere first deposited on amorphous Y₂O₃ surfaces, the base layer for IBADtexturing. For good texturing of MgO during the ion beam assisteddeposition (IBAD) it is important to have a smooth surface, which wasobtained by sequential chemical solution deposition of Y₂O₃ or Al₂O₃using acetate precursors dissolved in methanol. This process is calledsolution deposition planarization or SDP, and can produce surfaces assmooth as 0.5 nm RMS roughness when starting with a 50 nm RMS roughnessmetal foil substrate. The substrate was then placed in the vacuumdeposition system where MgO is deposited at a rate of about 3-5 Å/s andan Ar ion beam incident at 45° and with an ion energy of 700-1000 eV.Deposition typically takes about 10-30 seconds. Following IBAD a 50 nmthick film of homoepitaxial MgO was deposited in the vacuum chamber byevaporation at a substrate temperature of 400-700° C. During the IBADprocess as well as homoepitaxial deposition, film growth was monitoredby Reflection High Energy Electron Diffraction (RHEED) to verify thecrystalline alignment of the film. FIG. 2 shows RHEED images of the MgOfilm after IBAD and after homoepitaxial MgO. FIG. 3 shows an x-ray polefigure of the (202) peak demonstrating high degree of in-planealignment. FIG. 31 shows an XRD (101) pole figure of MOCVD-grown GaN.

In a second embodiment of the IBAD template, we utilized IBAD-texturedCeO₂ textured films. These IBAD films are oriented with the (111) axisout of the plane and form a 3-fold symmetric lattice on the surface,suitable for growth of hexagonal structure materials such as wurtziteGaN, hexagonally close packed (hcp) metals, or other 3-fold symmetric(111) buffer layers. IBAD-CeO_(x) films were grown on amorphous Al₂O₃surfaces, the base layer for IBAD CeO_(x). Other amorphous layers suchas SiO_(x) and Y₂O₃ are also suitable for IBAD-CeO_(x). Just as for MgO,to obtain the best texturing it is important to have a smooth surface,which was produced by sequential chemical solution depositionplanarization (SDP) of Y₂O₃ or Al₂O₃. With multiple coatings surfaces assmooth as 0.5 nm RMS roughness when starting with a 50 nm RMS roughnessmetal foil substrate can be achieved. The substrate was then placed inthe vacuum deposition system where CeO₂ was deposited at a rate of about3-5 Å/s by electron-beam evaporation and with an Ar ion beam incident at45° and with an ion energy of 700-1000 eV. Deposition typically tookabout 10-30 seconds. Following IBAD a 50 nm thick film of homoepitaxialCeO₂ was deposited in the vacuum chamber by evaporation. During the IBADprocess as well as homoepitaxial deposition, film growth was monitoredby Reflection High Energy Electron Diffraction (RHEED) to verify thecrystalline alignment of the film. FIG. 4 shows RHEED images of theCeO_(x) film after IBAD and after homoepitaxial CeO_(x).

Several different epitaxial buffer layers were deposited onIBAD-textured substrates by evaporation in vacuum. Although growth ofepitaxial GaN on CeO₂ is challenging due to the large lattice mismatchof the two crystalline lattices (GaN has a 16.7% smaller latticeconstant than CeO₂ (111)), by providing suitable intermediate bufferlayers one can transition or step-grade to a lattice match close to GaN.These intermediate buffer layers consisted of (111) metal oxides (suchas ZrO₂, Sc₂O₃, Y₂O₃), hexagonally close packed metals (such as Zr, Hf,Ti, Sc, etc.), and wurtzite or (111) metal nitrides (such as AlN, ZrN,TiN, etc.) During deposition of epitaxial buffer layers, film growth wasalso monitored by RHEED to verify the crystalline alignment. FIG. 4Bshows a RHEED image of epitaxial GaN grown on top of buffered IBAD. ForMOCVD GaN growth, several intermediate epitaxial buffer layerscomprising metal oxides, metal nitrides and elemental metals were used.For the IBAD-MgO template we successfully grew epitaxial GaN on γ-Al₂O₃(cubic aluminum oxide) and SrN. For the IBAD-CeO_(x) template GaN wasgrown successfully on epitaxial Hf and AlN.

FIG. 5 shows an x-ray diffraction GADDS 2D detector image thatdemonstrates a sharp GaN peak due to the single-crystal-like nature ofthe GaN film on top of the polycrystalline metal substrate. FIG. 6 showsthe (101) pole figure for the GaN grown on the 4-fold symmetric IBAD.The resulting GaN has a 12-fold symmetric (101) pole figure. This isbecause there are 2 different symmetric orientations of the hexagon on asquare lattice, resulting in two domains that are rotated with respectto each other by 30°. In contrast, GaN grows in a single domain on topof 3-fold symmetric IBAD such as (111) cube orientation. FIG. 7 shows a3-fold symmetric IBAD pole figure and FIG. 8 shows 6-fold symmetric GaNon the 3-fold IBAD-CeO. The in-plane full width half maximum of this GaNlayer is 1-2°. Out of plane rocking curve is 0.6-0.7° for GaN onIBAD/metal and 0.3° for IBAD/sapphire.

FIG. 9 shows several electron micrographs of epitaxial GaN layers on topof our 3-fold IBAD (111) template. This template includes a thin bufferlayer (less than 50 nm) of either Hf or AlN. Although in this case thecoverage of the GaN film is not complete due to incomplete coalescenceof grains, we can see very smooth (atomically smooth) surfaces on theGaN mesa structures. FIG. 10 shows optical images of epitaxial GaN grownon a double buffer layer structure that includes an epitaxial metal,such as Zr or Hf, together with an AlN layer grown by pulsed dcsputtering. GaN has also been grown on the AlN by reactive evaporationin this manner. This has so far yielded the best coverage of the GaNlayer by MOCVD. FIG. 11 shows a cross section of the film analyzed bytransmission electron microscopy where one can see the robustness of theIBAD and intermediate layers, as well as the smoothing of the SDPlayers. The epitaxial arrangement is preserved from the IBAD layer intothe GaN layer.

FIGS. 12 and 13 show RHEED in situ images of IBAD-CeO₂ samples,immediately after IBAD deposition and following homoepitaxial CeO₂ onIBAD-CeO₂, respectively. The images indicate single crystallineorientation as well as in-plane alignment. FIG. 17 shows the in-planealignment of the CeO₂ crystalline layer.

FIG. 15 shows a cross-sectional TEM micrograph of the complete structureincluding the thick GaN layer. One can see, from bottom, the rough metalfoil, the smoothing layers of the SDP planarization, followed by IBADand subsequent epitaxial buffer layers. The very top surface isextremely smooth enabling planar and other device fabrication.

FIG. 16 shows electroluminescence (EL) from an LED structure comprisingan InGaN multi-quantum well structure and a p-doped GaN layer on topthat was deposited epitaxially on top of the GaN film on the IBADtemplate on a metal foil.

FIGS. 18A-B show SEM micrographs of thick (about 5 micron) GaN films.Typically there are some defects, but also areas that are smooth over100 μm areas.

FIGS. 19A-B, show RHEED images of IBAD-Sc₂O₃ and homoepitaxial Sc₂O₃ ontop of IBAD-Sc₂O₃, respectively, with (111) orientation. Similar to CeO₂IBAD layers with (111) orientation can be obtained under the rightconditions, in this case the same conditions as for CeO₂ IBAD.IBAD-Sc₂O₃ texturing is formed on the amorphous SDP deposited Al₂O₃layers.

FIG. 20 shows the theta-2*theta scan of the IBAD-Sc₂O₃ film with ahomoepitaxial layer on top of the IBAD layer. The bright spot on theright is the (222) reflection of Sc₂O₃ and the ring on the leftrepresents the polycrystalline metal substrate. The main peak visible isdue to the (111)-oriented Sc₂O₃ material. FIG. 21 shows the pole figurefor the (440) pole of the Sc₂O₃ with 3-fold symmetry. These two x-rayscans demonstrate biaxial orientation for the Sc₂O₃ layer.

Example 2

A thick layer of GaN was deposited on an IBAD template. Typically GaN is4 to 6 micrometers in thickness, the top part of which is n-doped withSi, as can be seen in FIGS. 25A and 25B. FIG. 30 shows dislocations inthe structure. As shown in FIG. 26, on top of the GaN layer an LEDstructure pn junction is grown, together with a multi-quantum well (MQW)structure, which is a multilayer of 5 alternating InGaN and GaN layers.On top of the MQW is p-doped electron blocking layer followed by thep-GaN doped with Mg. Such a heterostructure is standard for making LEDsin industry.

Performance of such an LED device was measured as compared to LEDsprepared on single-crystal sapphire, which is a standard substrate inindustry. Results are shown in FIGS. 27-29. Comparisons were done withphotoluminescence (PL) (shining a light on the device), andelectroluminescence (EL) (passing a current through the device), lightmeasurements. For the first devices fabricated PL shows between 10 and40% of light in IBAD LEDs compared to standard sapphire LEDs. EL of IBADLEDs shows up to 12% (and increasing) of the sapphire LEDs. The ELdevice characteristics are dominated by leakage due to shorts in thecurrent devices, although the performance should improve significantlyas the material quality of the GaN layers improves.

Devices

Embodiments of the present invention are structures to improve certainaspects of, enable new functionalities in, and scale-up manufacturing ofGroup-III devices. In some embodiments, packaged InGaN LED devices aresimplified, thereby reducing their cost significantly. An embodiment ofthe present invention is a light-emitting LED device consisting of anInGaN p-n diode with a multiple-quantum-well (MQW) active region,fabricated directly on a metal foil substrate by use of an IBAD texturedtemplate prepared on the metal prior to GaN deposition. Such a device isshown in FIGS. 32A, 32B, 32C, and 34. FIGS. 33A and 33B show testresults for two such devices.

The present invention enables large-area deposition of Group-IIImaterials on flexible metal sheets. IBAD texturing enables high-qualityepitaxial materials can be deposited on appropriate metal substrateswhile approaching the performance of epitaxial materials onsingle-crystal substrates such as sapphire. FIG. 35 shows a schematic ofan LED fabricated on a metal foil substrate. The LED is preferablypre-packaged on the metal substrate and does not necessarily require atransfer to another substrate before application. For example,micro-LED's don't have to be transferred to a different substrate and/orbackplane. Conducting layers and phosphors can be printed on top of theLED metal foil sheets.

FIG. 36 shows an artist's rendition of a light emitting strip that canbe fabricated using the GaN-on-metal technology of the presentinvention. Instead of point sources of light that LEDs are typicallytoday, Light Emitting Strips (LESs) and Sheets become possible andeconomically feasible using the present invention. In this approachlight emitting areas are patterned over significantly larger areas,reducing the required current density. The edge of an LES preferablycomprises bus bars to carry higher currents along the length of thestrip. LESs have the advantage of being a continuous light source,compared to conventional LEDs which have a pixelated nature in thesource emitter. By spreading the light over a larger area, one alsoobtains a lower operating temperature, in addition to the benefitsprovided by the lower thermal resistance of the devices. A metal striplight source has the additional benefit of being flexible andconformable.

The GaN-on-metal approach for LEDs has many advantages when compared toGaN-on-sapphire, including:

-   -   A flexible substrate enables roll-to-roll processing (R2R). R2R        in turn enables scaled up manufacturing and implementation of        printed electronics technologies. It is easy to scale        manufacturing from 6-inch substrates to km-long webs of metal        foil. This scale up alone will yield significant reduction in        cost.    -   Better thermal conduction of the substrate and uniformity in        production should ultimately result in higher manufacturing        yields.    -   The coefficient of thermal expansion (CTE) of the metal        substrate is better matched to that of GaN. CTE of molybdenum        metal is 5.5×10⁻⁶/K, compared to GaN of 5.6, a 2% mismatch which        can be further reduced by use of a molybdenum-copper alloy.        Sapphire CTE is about 7.5, causing a significant bowing of        substrate upon cool down after high temperature growth.    -   There is potentially a better lattice match of the substrate        with GaN, once better lattice match materials are implemented in        IBAD. Sapphire has a 16% lattice mismatch. The IBAD material,        CeO₂ is also 16%, but other materials that are known to be        amendable to IBAD texturing are only a few % mismatched to GaN.    -   Simpler packaging of LEDs is due to the use of metal substrate        as both a reflector and a heat sink for the LED. With typical        LEDs, a reflector metal is deposited on a roughened surface of        the LED in backend processing. A rough surface is preferred        since the light needs to bounce a few times before exiting the        LED. Also, in the course of packaging the LED is bonded to a        separate heat sink. In contrast, in embodiments of the present        invention the starting metal substrate preferably has a rough        surface and is used as a built-in reflector for the light        emitted by the LED. The metal substrate such as molybdenum can        additionally be coated with a higher reflectivity material for        better reflectance at shorter wavelengths, where InGaN LED        emitters are most efficient. The metal substrate furthermore        preferably has a high enough thermal conductivity (142 W/m·K for        molybdenum, compared to sapphire at 25 W/m·K) and thickness to        be an effective heat sink for heat generated by the LED, without        the need for substrate removal even at high power requirements.        Thus, with some embodiments of LEDs of the present invention, no        external heat sink or reflector is required.    -   The simplified packaging coupled with roll-to-roll processing of        devices, among other aspects of applying phosphors and other        down converters and printing of contacts, such as screen        printing and laser etching, yield significantly lower costs for        LEDs. Cost per unit area is expected to be >10× lower once        roll-to-roll processes are implemented.    -   The ability to reduce droop due to larger area LED devices is        possible. Since the cost of epi area is reduced, larger areas        can be utilized, thus reducing current densities in operating        LEDs. This in turn will increase efficiency of devices by        reducing the droop.    -   A reduction in thermal droop can be achieved due to the        reduction in thermal resistance by utilizing a thin metal        substrate for the LED device. It is estimated that the operating        temperature of LEDs on metal will be about ½ that of LEDs on        sapphire. Thus, even without a heat sink, high brightness LED's        will be cool to the touch, unlike current ones.    -   More robust operation and longer lifetimes can be achieved due        to lower temperature of operation because of better thermal        resistance. This is especially applicable to light        downconverters, such as phosphors, that are placed on LEDs.    -   Large-area GaN sheets also enable monolithic integration of        various GaN-based devices, such as LEDs and power transistors,        to control the current in the LEDs.    -   GaN power devices, such as HEMT, will also benefit from some of        the features described above, in particular better thermal        management and scaled up manufacturing.

Although the invention has been described in detail with particularreference to the disclosed embodiments, other embodiments can achievethe same results. Variations and modifications of the present inventionwill be obvious to those skilled in the art and it is intended to coverall such modifications and equivalents. The entire disclosures of allpatents and publications cited above are hereby incorporated byreference.

What is claimed is:
 1. An electronic or optoelectronic device comprisinga substrate on which an active region of the device was grown, saidsubstrate acting as a reflector, the device further comprising anepitaxial hexagonal crystal layer and a layer of a cubic material havinga <111> out of plane orientation and having an in-plane crystallinetexture with a full width half maximum (FWHM) less than or equal toapproximately 15°; wherein the layer of cubic material has an in-planecrystalline texture having a FWHM of less than or equal to approximately12°.
 2. The device of claim 1 selected from the group consisting of LED,MOSFET, MESFET, HEMT, Heterojunction FET, heterojunction bipolartransistor (HBT), thin-film transistor, sensor, memristor, laser diode(LD), SAW device, spintronic device, photodetector, and photovoltaic(PV) diode.
 3. The device of claim 1 wherein said substrate is a heatsink.
 4. The device of claim 3 wherein said substrate has a thermalconductivity greater than approximately 25 W/m·K.
 5. The device of claim4 wherein said substrate has a thermal conductivity greater thanapproximately 50 W/m·K.
 6. The device of claim 1 wherein said substratecomprises a metal or alloy.
 7. The device of claim 1 wherein saidsubstrate is flexible.
 8. The device of claim 1 wherein said substrateis not a single crystal.
 9. The device of claim 1 comprising a lightemitting region which is two-dimensional and not one or more pointsources.
 10. The device of claim 9 in the form of a sheet or strip. 11.The device of claim 1 comprising an LED on a metal substrate, wherein anoperating temperature of the LED is less than approximately two thirdsthat of an LED on sapphire.
 12. The device of claim 11 wherein anoperating temperature of the LED is less than approximately one halfthat of an LED on sapphire.
 13. The device of claim 1 that is cool totouch during operation.
 14. The device of claim 1 integrated into anelectronic system.
 15. The device of claim 14 wherein said electronicsystem is selected from the group consisting of an LED-based luminaire,a light emitting strip, a light emitting sheet, an optical display, anda MicroLED display.
 16. The device of claim 1 wherein the epitaxialhexagonal crystal layer comprises a group III-nitride semiconductor. 17.The device of claim 16 wherein the epitaxial hexagonal crystal layercomprises GaN.
 18. The device of claim 1 wherein the layer of cubicmaterial has been textured by ion beam-assisted deposition (IBAD). 19.The device of claim 1 wherein the layer of cubic material is selectedfrom the group consisting of MgO, CeO₂, a bixbyite structure, Sc₂O₃,Y₂O₃, Al₂O₃, a fluorite structure, TiN, a rock salt structure, CaF₂,cubic ZrO₂, HfO₂, ScO_(x), and Mn₂O₃.
 20. The device of claim 1 whereinthe epitaxial hexagonal crystal layer comprises GaN and the substratecomprises molybdenum, tungsten, tantalum, alloys thereof, Mo—Cu, or TZM.21. The device of claim 1 comprising a base layer disposed between thesubstrate and the layer of cubic material.
 22. The device of claim 21wherein the base layer comprises amorphous Al₂O₃, Y₂O₃, or SiO₂.
 23. Thedevice of claim 1 comprising one or more epitaxial buffer layersdisposed between the layer of cubic material and the epitaxial hexagonalcrystal layer.
 24. The device of claim 23 wherein the epitaxial bufferlayers each have a lattice parameter that successively provides atransition from the lattice parameter of the cubic material to thelattice parameter of the epitaxial hexagonal crystal layer.
 25. Thedevice of claim 24 wherein the epitaxial hexagonal crystal layercomprises GaN and the epitaxial buffer layers comprise a layer of Sc₂O₃,a layer of Zr, and a layer of AlN.
 26. An electronic or optoelectronicdevice comprising a substrate on which an active region of the devicewas grown, said substrate acting as a reflector; wherein said substrateis a heat sink.
 27. The device of claim 26 selected from the groupconsisting of LED, MOSFET, MESFET, HEMT, Heterojunction FET,heterojunction bipolar transistor (HBT), thin-film transistor, sensor,memristor, laser diode (LD), SAW device, spintronic device,photodetector, and photovoltaic (PV) diode.
 28. The device of claim 26wherein said substrate has a thermal conductivity greater thanapproximately 25 W/m·K.
 29. The device of claim 28 wherein saidsubstrate has a thermal conductivity greater than approximately 50W/m·K.
 30. The device of claim 26 wherein said substrate comprises ametal or alloy.
 31. The device of claim 26 wherein said substrate isflexible.
 32. The device of claim 26 wherein said substrate is not asingle crystal.
 33. The device of claim 26 integrated into an electronicsystem.
 34. The device of claim 33 wherein said electronic system isselected from the group consisting of an LED-based luminaire, a lightemitting strip, a light emitting sheet, an optical display, and aMicroLED display.
 35. An electronic or optoelectronic device comprisinga substrate on which an active region of the device was grown, saidsubstrate acting as a reflector; wherein the device comprises a lightemitting region which is two-dimensional and not one or more pointsources.
 36. The device of claim 35 selected from the group consistingof LED, MOSFET, MESFET, HEMT, Heterojunction FET, heterojunction bipolartransistor (HBT), thin-film transistor, sensor, memristor, laser diode(LD), SAW device, spintronic device, photodetector, and photovoltaic(PV) diode.
 37. The device of claim 35 wherein said substrate comprisesa metal or alloy.
 38. The device of claim 35 wherein said substrate isflexible.
 39. The device of claim 35 wherein said substrate is not asingle crystal.
 40. The device of claim 35 in the form of a sheet orstrip.
 41. The device of claim 35 integrated into an electronic system.42. The device of claim 41 wherein said electronic system is selectedfrom the group consisting of an LED-based luminaire, a light emittingstrip, a light emitting sheet, an optical display, and a MicroLEDdisplay.